a) Field of the invention
The present invention relates to a compact vari-focal lens system for lens shutter cameras which comprises a small number of lens elements and has a high vari-focal ratio.
b) Description of the prior art
In the recent years, the prevalence of compact cameras has produced demands for vari-focal lens systems which are compact, light in weight and have high-grade specifications. Especially for use with the lens shutter cameras which do not permit exchange of lens systems, it is demanded to develop compact vari-focal lens systems having vari-focal ratios of 2 and higher.
As a vari-focal lens system which is designed so as to satisfy the demand for compact and light-weight design, there is already known a lens system which consists of a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power, and is adapted so as to perform variation of focal length by varying the airspace reserved between these two lens units. However, it is difficult to design this vari-focal lens system so as to have a vari-focal ratio of 2 or higher and high-grade specifications since the focal length of this lens system is varied by moving the rear lens unit which has a magnification higher than 1.times. and the negative refractive power.
As vari-focal lens systems which have solved this problem or are designed so as to have high vari-focal ratios, there are known the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 63-43115 and Japanese Patent Kokai Publication No. Hei 1-252916. Each of these vari-focal lens systems consists, in the order from the object side, of three positive, positive and negative lens units or four positive, negative, positive and negative lens units, and has a vari-focal ratio on the order of 3. However, this vari-focal lens system comprises at least eleven lens elements even when aspherical surfaces are used therein and is not sufficiently satisfactory from the viewpoint of compact and lightweight design.
On the other hand, attentions are now paid to the lens systems which adopt graded refractive index lens elements. Graded refractive index lens elements are classified, dependently on directions of refractive index distributions, into the axial type which has a refractive index distribution in the direction along the optical axis, the radial type which has a refractive index distribution in the direction perpendicular to the optical axis, and so on. Especially the radial type graded refractive index lens element is expected to exhibit an effect to make lens systems compacter and lighter in weight.
The lens systems disclosed by Japanese Patent Kokai Publication No. Sho 63-159818 and Japanese Patent Kokai Publication No. Sho 63-161423 are known as the vari-focal lens systems for lens shutter cameras having vari-focal ratios of approximately 3 and using graded refractive index lens elements. Each of these lens systems is designed so as to suppress variations of aberrations during variation of focal lengths by using the axial type graded refractive index lens element, but comprises eleven lens elements and is not sufficiently satisfactory from the viewpoint of the compact and lightweight design.
Further, the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 61-126515, Japanese Patent Kokai Publication No. Sho 61-148414 and so on are vari-focal lens systems for lens shutter cameras using the radial type graded refractive index lens elements and comprising lens elements in small numbers, the minimum of which is only five. However, these lens systems have low vari-focal ratios of 1.5 to 2 and are still unsatisfactory from the viewpoint of the high-grade specifications.